The present applicant proposed in Japanese Patent Publication (A) No. 2007-303423 a spark ignition type internal combustion engine which is provided with a variable compression ratio mechanism which can change a mechanical compression ratio and with a variable closing timing mechanism which can change a closing timing of the intake valve and which raises the mechanical compression ratio to 20 or more at the time of engine low load operation compared with at the time of engine high load operation.
In this spark ignition type internal combustion engine, at the time of engine low load operation, by making the mechanical compression ratio (expansion ratio) 20 or more and by making the closing timing of the intake valve a timing away from suction bottom dead center, the actual compression ratio is maintained comparatively low compared with the mechanical actual compression ratio, the occurrence of knocking due to the rise in the actual compression ratio can be suppressed, and an extremely high heat efficiency can be realized.
In this regard, if retarding the closing timing of the intake valve so as to become separate from suction bottom dead center so as to reduce the amount of intake air into a combustion chamber, part of the intake gas which is taken into the combustion chamber once is pushed out by the rising piston and is blown back into the engine intake passage. The amount of intake gas blown back into the engine intake passage becomes greater the more the closing timing of the intake valve is retarded. Further, the strength of the intake gas blown back into the engine intake passage becomes greater the more the closing timing of the intake valve is retarded. In the spark ignition type internal combustion engine which is described in Japanese Patent Publication (A) No. 2007-303420, sometimes the closing timing of the intake valve is retarded to an extreme extent. In such a case, the amount of blowback of intake gas becomes extremely large or the strength of blowback of the intake gas becomes extremely great.
In this way, under conditions where the blowback of intake gas is large and strong, in particular, at the time of engine cold start, deviation occurs in the air-fuel ratio between cylinders (ratio of fuel to air in the combustion chambers).
That is, at the time of engine cold start, the fuel which is injected from a fuel injector is difficult to atomize (or vaporize, below, “atomization” including “vaporization”) and parts where the fuel concentration is high and parts where the fuel concentration is low end up being formed in the air-fuel mixture in some cases. However, if the blowback of the air-fuel mixture into the engine intake passage is slight and weak, the blown back air-fuel mixture will not be taken into other cylinders, but will again be taken into the cylinder in question at the next cycle. For this reason, not much deviation occurs in the air-fuel ratio between cylinders.
On the other hand, if the blowback of the air-fuel mixture into the engine intake passage is large and strong, part of the blown back air-fuel mixture will be taken into an adjoining cylinder etc. As explained above, at the time of engine cold start, parts where the fuel concentration is high and parts where the fuel concentration is low are formed in the air-fuel mixture in some cases. In this case, if the air-fuel mixture which is blown back to the inside of the engine intake passage and is taken into an adjoining cylinder is a part with high fuel concentration, the air-fuel ratio of the air-fuel mixture which is taken into this adjoining cylinder etc. will become richer than a target air-fuel ratio while the air-fuel ratio of the air-fuel mixture which is taken into the original cylinder will become leaner than the target air-fuel ratio. Conversely, if the air-fuel mixture which is blown back to the inside of the engine intake passage and is taken into an adjoining cylinder is a part with low fuel concentration, the air-fuel ratio of the air-fuel mixture which is taken into this adjoining cylinder etc. will become leaner than a target air-fuel ratio while the air-fuel ratio of the air-fuel mixture which is taken into the original cylinder will become richer than the target air-fuel ratio. For this reason, deviation will end up occurring in the air-fuel ratio among cylinders.
In this way, at the time of engine cold start, if retarding the closing timing of the intake valve, the blowback of the air-fuel mixture into the engine intake passage becomes greater and stronger whereby deviation will end up occurring in the air-fuel ratio among cylinders.